Glucose is a simple sugar that can be used in a variety of food, feed, and chemical applications. However, glucose availability and cost have recently become a limiting factor in the demand for an inexpensive biofuel feedstock and sustainable animal feed. Demand for corn and sugarcane has increased the price of this commodity significantly. Starch is a large polymer composed of repeating glucose residues linked via α-1,4 and α-1,6 bonds (Stitt & Samuel C Zeeman 2012). Starch is a superior source of glucose because of it's simple molecular structure (α-1-4, and α-1-6 glucose linkages) and the relative ease with which these bonds are accessed and hydrolyzed by inexpensive and highly effective enzymes (e.g.; α-amylase and glucoamylase). Starch derived from plant materials can be readily converted into glucose either in the digestive tracts of animals (feed applications) or biochemically (for example, via acid hydrolysis or enzymatic hydrolysis). Hydrolysis of high-starch plant tissues like grain provides relatively pure glucose that is effectively transformed into meat or chemical end-products.
Glucose can also be derived from other plant-produced polymers, such as cellulose, β-glucan, or xyloglucans. However, the processes for releasing the glucose from these polymers are generally much less efficient; they are less easily digested by ruminant and monogastric animals, and chemical means of releasing the glucose typically involve harsh chemical treatments followed by hydrolysis with expensive enzyme cocktails (Alvira et al. 2010).
Sucrose, a soluble storage carbohydrate, is also a plant derived feedstock molecule that is readily utilized by fermentative organisms. Cropping and processing systems that use sucrose feedstocks, such as sugarbeets and sweet sorghum, are constrained by narrow harvest windows and storage and stability limitations. Sweet sorghum must be processed similarly to sugarcane, within days of its harvest to limit microbial fermentation of the sucrose due to the high moisture content in the harvested materials (spoilage). Campaign periods reduce the overall capital effectiveness of dedicated processing facilities.
Lignocellulosic substrates are less attractive feedstocks because of processing difficulties. Lignocellulosic biomass contains a mixture of hexoses and pentoses and their recalcitrance to hydrolysis (crystallinity, and cross-linking to lignin) makes digestion and cost effective degradation into useable sugars difficult. In biofuels production, expensive pretreatments are being developed to aid in complete hydrolysis of lignocellulosic materials. Full utilization of the resultant mixtures of sugars for fuel and chemical production also requires that specialized fermentation organisms transform the resulting sugars into final products, such as ethanol, butanol, succinic acid, and other chemicals.